1. Field
The present disclosure relates generally to telecommunications, and more particularly to coherent initial acquisition of a pilot signal.
2. Background
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation technique, which effectively partitions the overall system bandwidth into a large number of sub-carriers. These sub-carriers are spaced apart at precise frequencies to provide orthogonality. Content may be modulated onto the sub-carriers by adjusting each sub-carrier's phase, amplitude or both. Any suitable conventional modulation scheme may be used, such as phase shift keying (QPSK) or quadrature amplitude modulation (QAM) just to name is used, but other modulation schemes may also be used.
In OFDM systems, content is generally broadcast in super frames. A super frame generally consists of a number of symbols that are OFDM modulated with (N+K) sub-carriers, wherein the content is modulated onto N sub-carriers and a cyclic prefix is modulated onto K sub-carriers. The first OFDM symbol in the super frame is a pilot signal. This pilot signal is generated with a pseudo-random (PN) number that is repeated multiple times across the OFDM symbol. The time domain periodic structure of the pilot may be exploited to detect the start of the OFDM super frame and correct the initial frequency offset between the transmitter frequency and the local receiver frequency.
Heretofore, an incoherent acquisition process was used to process the pilot signal to detect the start of the super-frame and correct the initial frequency offset. Incoherent acquisition is a process whereby a delayed version of the received signal is correlated with the received signal itself. The correlator output has a high value during the pilot signal due to its periodicity. This approach, however, lacks the ability to differentiate the signal from tone interference well. In addition, the estimation of the initial frequency offset may be significantly impacted by the presence of tone interference.
Accordingly, there is a need in the art for improvements in the processes currently used for frame synchronization and initial frequency offset estimation in OFDM systems that are also applicable to other telecommunication systems employing a periodic pilot signal.